Rendering

Rendering fat is simple, if unpleasant.  Start with two pounds of raw fat for every pound of fat called for in your soap recipe.  Cut it into small pieces, and put on low heat until it has melted.  There will be small pieces of meat (cracklings) left - skim these out (eat them or feed them to your cat or dog) or strain the fat.

A couple of tips:  I wouldn't want to do this in my kitchen - it takes a long time, and it smells really awful.  Outside is best, but I've done it in a turkey roaster on an electric burner in the basement.

 Drippings

I normally use grease and drippings instead of rendering, not only because it's not as unpleasant, but also because it's free.  Save all of your grease (I put it into a metal coffee can).  When your container is full, put it in a large pot with an equal amount of water and 2 tablespoons of salt.  Heat it up, then add 1 quart of cold water for every gallon of liquid, and allow it to cool.  It will separate into three layers with the pure fat on top - skim this off.  You will probably need to rinse it, too, and I have sometimes done this a second time if it comes out with too many impurities.

If the grease is rancid, substitute 1 part vinegar and 5 parts water for the water.  You can also cook a sliced potato in the fat to deodorize it.

I don't make soap very often, so I just stick the fat in a gallon freezer bag and put it into the refrigerator (you could freeze it, I suppose, though I never have) until I'm ready to use it.